The invention concerns powder slurry coating compositions and processes for preparing such compositions.
Powder coating technology avoids the regulated emissions from using solventborne coating compositions or waterborne coating compositions, which have some volatile organic content. Powder coatings, however, have unique technological challenges that result from the absence of a liquid medium, especially in preparing coatings that require high gloss or smoothness. One problem encountered with powder coatings is balancing the need for reducing the median particle size of the powder to obtain smoother coating films with the need to avoid a significant fraction of particles below about five microns that may present a dusting hazard in handling and applying the powder coating. Another problem is sintering of the powder particles during storage. Finally, it can be difficult to control film build in applying powder coatings even for fluidized methods because of variation in density of the powder in the air.
Recently, powder slurry technology has been used as a means for delivering powder coatings with finer median particle sizes or with a more significant fraction of low particle size material, while avoiding the dusting problem. In general, powder coatings have been prepared by first melt mixing all of the coating ingredients, typically including one or more solid resins and one or more pigments, in an extruder. The extrudate is then flaked and pulverized to the desired median particle size or particle size distribution. To prepare a powder slurry, a coarse powder coating powder with a larger particle size is dispersed in water, usually with one or more additives such as surfactants, dispersants, and thickeners. The dispersion is then milled to further reduce the median particle size of the dispersed powder. This method typically results in powder coating slurries that have a median particle size of about three to four microns or more. The water medium also keeps the particles separated and consequently avoids sintering of the particles.
In a similar process, U.S. Pat. No. 5,379,947 discloses a powder slurry coating with a particle size distribution in which at least half of the particles are between three and five microns and all of the particles are less than about ten microns. Because the powder particles are in the form of an aqueous slurry instead of a dry powder, the slurry avoids the dusting problem and associated problem of fine airborne particulates. The powder slurry is produced by first melt mixing all of the coating ingredients in an extruder and reducing the extrudate to a flake. The flake is then jet milled to a fine powder of the desired particle size distribution. The powder is dispersed in water with surfactant, dispersant, and thickener to make the powder coating slurry. While the powder slurry composition of U.S. Pat. No. 5,379,947 beneficially provides small median particle sizes, it is a lengthy process and the very fine jet-milled powder is difficult to disperse.
It would be desirable to reduce the time and cost involved in producing powder coating slurries. The step of preparing the melt extrusion is time-consuming and uses expensive equipment that is difficult to clean.
We have now discovered a simplified method of producing a powder slurry coating composition that matches the performance of powder slurry coating compositions produced by the old melt extrusion method.
The invention provides a process for preparing a powder slurry coating composition that includes steps of:
(a) providing individual particulate materials including at least one resin powder and at least one further particulate material;
(b) dispersing the individual particulate materials in an aqueous medium; and
(c) milling the dispersed particulate materials to a maximum particle size of up to about 15 microns.
Two or more of the particulate materials may be mixed together before being added to the aqueous medium. The materials are not mixed on the sub-particle scale, as is done in the extruder melt mixing method, but instead each particle retains its individual identity. The particulate material added to the aqueous medium is thus heterogeneous. The milling of the dispersed particulate materials is thought to homogenize the dispersed particulate to some degree, as well as reduce the overall median particle size. In another aspect, the invention provides a powder slurry coating composition prepared according to the above process.
The powder slurry coating composition is used to prepare a coating on a substrate. The powder slurry coating composition can be applied to various substrates, including especially metal and plastic substrates. The powder coating may be applied to many different articles, such as, without limitation, furniture, wheels, appliances, tools and power tools, industrial and automotive parts, and containers. In one example, the powder slurry coating composition may be formulated for exterior vehicle applications, including primer coatings (also called filler coatings) and topcoat coatings, including basecoat and clearcoat coatings. The coatings prepared from the powder slurry coating compositions of the invention are equivalent in appearance to coating prepared by the melt-mix extrusion method, while having the advantage of being simpler and less expensive to manufacture.